There are two methods I am aware of to input cheat codes, mainly relating to Windows Vista. 1) you must press "Ctrl - Shift - C" or 2) "Ctrl - Shift - Win - C". (Win = the start menu key usually between the "Ctrl" and "Alt" keys.
If the first option doesnt give you the cheats bar, the second one should. Note: you must press the keys at the same time during game play to enable it.
Hope this helps!